[PATCH] D80858: [CUDA][HIP] Support accessing static device variable in host code

[Yaxun Liu via Phabricator via cfe-commits]

yaxunl added a comment.

In D80858#2171266 <https://reviews.llvm.org/D80858#2171266>, @hliao wrote:

> We may try to solve the issue without RDC firstly, where we don't need to change that static variable name (if the runtime maintains the device binaries correctly.) We only need to ensure the linker won't remove their symbols.


This is essentially externalizing it in linker. However, it may not work.

Consider a static device var was accessed in host code through hipMemCpyToSymbol or hipMemCpyFromSymbol. In device code, this static var may be initialized through host code, or its final value may be read by host code after kernel execution. The existence of these operations mean that this static device variable `is` actually having `external` linkage, instead of `internal` linkage, since it is accessed by external modules. Fail to reflect this truth in IR will cause optimization passes to make incorrect assumptions about this variable and perform incorrect optimizations on it. e.g. the llvm passes can assume the value in this static var never changes, or its final value will not be used, then the llvm passes may simply remove it. Marking it as `used` will not solve the issue, since the llvm passes may still assume its value never changes after initialization, whereas in reality it may be changed by hipMemCpyToSymbol before kernel execution.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D80858/new/

https://reviews.llvm.org/D80858